I am on record calling this phrase “The DUMBEST phrase we use in Transfusion Medicine” from the stage at an AABB Annual Meeting. That seems like a pretty harsh assessment (and I may be wrong), but I really don’t like “least incompatible.” Let me explain.
When a patient presents with warm autoantibodies, it is usually impossible to find red cell units that are crossmatch-compatible, because the autoantibody reacts against not only the patient’s OWN cells, but pretty much everyone ELSE’S cells as well. As a result, once we make sure there are no alloantibodies using techniques such as autoadsorption, we usually have to issue a red cell product that is crossmatch-incompatible. When we do this, the laboratorian typically tests the patient’s serum/plasma against multiple different ABO- and Rh-compatible red cell units, and chooses the one that shows the weakest reactions (therefore, is “least incompatible” with the donor…I guess). This means that, for example, if the patient’s plasma reacts at 3+ against donor 1’s RBCs and 2+ against donor 2’s, the transfusion service will usually choose to issue donor 2’s RBCs.
The problem, for me, is not only that the “least incompatible” phrase is an awkward double-negative (why on earth isn’t it “most compatible?”), but also that it doesn’t mean anything! There is no evidence, to my knowledge, that choosing red cell units that react less strongly with an autoantibody compared to other units improves post-transfusion survival of those red cells! Worse, the term is often used as false reassurance to our transfusing staff (I’ve heard people say things like, “Yes, doctor, the crossmatch is incompatible, but, by golly, we are going to give you the LEAST INCOMPATIBLE units!”). Those words may sound good, but it is a mistake to use them to try to reassure someone of the safety of what you are doing. By the way, if you’ve done work to rule out alloantibodies, as mentioned above, giving incompatible RBCs generally IS safe, but NOT because we’ve chosen the “least incompatible” unit!
I don’t mind blood bankers using this silly phrase to each other, but I really do have a major problem with using it to reassure anyone outside of our blood banks. Further, I acknowledge that choosing a unit that reacts less strongly with patient autoantibodies over another that reacts more strongly is reasonable (though again, there’s no proof it is safer).
But don’t just take my word for it! The great Dr. Larry Petz, all the way back in 2003, argued that we should discard this phrase (or at least, to stop using it without a scientific definition). I can’t say it any better than he did in the last paragraph of the article linked above:
“Although least incompatible unit is a term that is entrenched in ‘in‐house’ jargon, it should not be used in discussions with attending physicians and, if used at all, should not be permitted outside the confines of the transfusion service. It is time for transfusion service personnel and clinicians to discuss issues surrounding transfusion of patients with AIHA in informative, scientific ways.” Lawrence D. Petz, MD, in Transfusion 2003
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